Endothelium is one of the principal tissues of an organ transplant and it is not known whether endothelium is immunogenic. Passenger cell depletion and prolongation of the allograft survival has been achieved by the use of the interim host system with the athymic immuno-incompetent mouse (nude). This system has previously been used for the study of immunogenicity of non-vascularized tissue but has not been used for the study of vascularized whole organ grafts. In this proposal, a vascularized whole organ graft of the rat heart will be studied for endothelial immunogenicity using a new heart transplant model developed for this purpose utilizing the athmyic immuno-incompetent rat as the interim host. This model does not have the difficulties of organ culture techniques and has a clear endpoint of cessation of contractions. Information on endothelial immunogenicity may be useful in the pretreatment of human organ grafts prior to transplantation.